1. Field of the Invention
The present invention relates to a piezoelectric resonator and an electronic component provided therewith and, more particularly, to a technology effective to achievement of a wider bandwidth of a passing property in a piezoelectric resonator having stacked transducers.
2. Related Background of the Invention
In recent years there are increasing demands for compact, low-loss, and wide pass bandwidth filters in order to keep up with fast large-volume communication. Filters meeting such demands and frequently used are Surface Acoustic Wave (SAW) filters with compact and low-loss characteristics, and antenna duplexers using the SAW filters. However, operating frequencies are becoming higher from demands for faster larger-volume communication.
A SAW filter is a filter for exciting and receiving a surface acoustic wave with use of interdigital electrodes in which electrode fingers in the width of about a quarter of the wavelength λ of the propagating surface acoustic wave are alternately placed on a piezoelectric substrate. The line width of the electrode fingers of the SAW filter used in 2-GHz band systems is approximately 0.4 μm. For supporting achievement of higher frequencies of this SAW filter, it is necessary to precisely process the electrode fingers of not more than 0.4 μm. Therefore, there is a high possibility of considerable degradation of productivity.
A SAW filter is disclosed, for example, in Japanese Patent Application Laid-Open No. 5-167388.
Under such circumstances, attention is drawn to transducers with a piezoelectric thin film being a kind of devices using the Bulk Acoustic Wave (BAW devices), for example, as described in Japanese Patent Application Laid-Open No. 2003-22074. The operating frequency of the BAW devices is determined by the thickness of the piezoelectric film interposed between input and output electrodes.
In passing, the conventional transducer filters using ceramics or quartz have not been used for high-frequency usage, because it is difficult to precisely process the thin piezoelectric film. On the other hand, in the case of thin-film transducer filters, the piezoelectric film can be formed by means of a film-forming system of sputtering or the like; therefore, it is easy to obtain the piezoelectric film in a desired thickness. The thin-film transducer filters thus have advantage in achievement of higher frequencies. The electrodes used in the thin-film transducer filters are flat-plate electrodes. Since the thin-film transducer filters do not have to use the thin electrodes as used in the SAW filters, high-power signals can be handled therein accordingly.
A piezoelectric resonator comprised of a conventional stack type transducer is shown in FIG. 9.
The conventional piezoelectric resonator 40 shown in FIG. 9 is composed of a substrate 11, for example, of silicon, an acoustic multilayer reflecting film 12 disposed on the substrate 11, and a transducer 22 disposed on the acoustic multilayer reflecting film 12. The acoustic multilayer reflecting film 12 has first reflecting films 12a and second reflecting films 12b alternately stacked. The first reflecting films 12a and the second reflecting films 12b have their respective acoustic impedances different from each other. The transducer 22 has a first electrode film 20a, a piezoelectric film 21, and a second electrode film 20b stacked in order. A plurality of such transducers 22 in the structure in which the piezoelectric film 21 is sandwiched between the upper and lower electrode films 20a, 20b, are two-dimensionally arranged and the transducers 22 are electrically connected in a ladder configuration, thereby constituting a filter or a duplexer.